Telephone transmission system



Patented J an. 1932 KENNETH E.LATIME R, on ALnWYcH, LONDON, ENGLAND,ASSIGNORTO "WESTERN ELEC- rarc COMPANY, rncoaroan'rrn, or NEW YORK YORK,N. Y., A CORPORATION .OFNEW TELEPHONE rnA srrrssIoN SYSTEM Applicationfiled, February 28, 1930, Seria1 No. 432,203, and. in Great BritainMarch 11, 1929.

This invention relates to telephone transmission systemsand-n1oreparticularly to methods of loading long transmission'lines toavoid transient distortion and echoes.

' British Patent 181,328 gives the theory 10 the said patent that theloadingmust be reduced as the length of the lineis increased. Thepresent invention may be regarded as a development of the aboveprinciples in order to malretheir application more practical andeconomical. f

Over any large" and populous areasuch asin the large European countriesat least two or three different types of cable are required toaccommodate economically the type to give satisfactory transmission upto a diii'erent limiting length of cable. 0 In other words, cables of atleast two or three different transmission distance ranges are required.

should be chosen as standard andthe "manner in which circuits of thechosen ranges should be loaded, having regard to the quality andefficiency of transmission.

It was found from considering the conditions represented for example bythe more important European countries and considering the manner inwhich circuits of required lengths may be loaded. that the cost of anadequate system of circuits depends very considerably on thestandardranges ofcir cuits and their loading. This will be made moreclear fromaconsideration ofthe statistics which will be givenhereinafter.

a large number of different lengths which vary over a wide range. areloade'dfwith a give minimum cost of the system'as. a whole, for agiven'grade of transmission.

According to one feature of the invention transmission of telephonicmessages, each The problems encountered are what ranges In accordancewith the invention. cables of a loaded telephone transmission systemcomprises two types of loaded cable circuits whereinthe high gradecircuits have double the loading spacing of the other circuits. Thus theloading coils of the high grade or long distance circuits may bedisposed at the alternate loading points on the lower grade or shortdistance circuits.

According to another feature a telephone transmission system comprises aplurality of circuits loaded with different loading spacings wherein theloading coil inductance is varied as the reciprocal of the loadingspacings. I

It has been'found'that supposing w, and 2 to be the-costs per mileofcircuit of three types of circuit, and a, b, and 0 to be the per-.

centage of the total miles of circuit lying.

between the ranges from zero to X, from X" to Y,and from Y to 3000miles, substantially similar values represent the cost of maintenance;and'according to a feature of the invention, by suitable lengths ofcircuits and suitable loading design thesecosts are made a minimum for asion efiiciency. I

For instance, circuits bearing a ratio of ideally 1:2.2z5 orapproximately 1:22:42 are economical standards and in this case theloading spacing of the longest circuits may be advantageously doublethat of the shorter range circuits. For example, suitable conditions aremade by three types Whose ranges are 250. 550 and 1250 milesrespectively and the loading spacing for the 1250 mile circuits would betwice that of the 250 mile circuits. The loading spacing may bestaggered.

Fig. l 'is a curve for a system of circuits, connecting the numberofcircuits to be provided and the maximum distances'over which they mustgive satisfactry transmission,

Fig. 2 gives curves with range or length as-abscissaand cost per mile ofcircuit as ordinates.

-Fig. 3 shows curves relating relative cost of 100 circuits. per mile,(as ordinates) and range or length as abscissasl, and is used toillustratefa principle of the present invention.

given standard of transmis- Fig. 4 shows curves connecting total annualcost per 100 circuits per mile as ordinates with range as abscissa.

Fig. shows curves similar to Fig. 3 except that the ordinates representrelative annual costs for 100 circuits.

From the Formula 4 given in the above mentioned patent, it will beobserved that the transients depend upon the time of direct transmissiont and a function involving the cut-off frequency and the highestfrequency to be transmitted whereas the echoes are dependent solely uponthe time of direct transmission. From this, it is apparent that if t iskept constant there is no need to change the cut-off frequency.Therefore, if some method can be devised for raising the velocity oftransmission without affecting the cut-off frequency, this would beideal from an economical aspect since it is usually accepted that thecost of a circuit of given velocity is partly dependent upon its cut-oilfrequency.

In contemplating the practical application of the above principles itwas found that by choosing lines of considerably different lengths andinstead of maintaining a uniform loading spacing designing the coils andcoil spacing according to said formula, it was possible to economize inloading coils. Thus, if lines of say 500 mile lengths have a coilspacing of 8, lines with a length of say 1000 miles may have a coilspacing of 28, the inductance coils of the latter case beingapproximately half the value of those in the former case.

The following statistics reveal how it is possible to determine the typeof system most suitable for telephonic conditions.

1 shows the relation between the number of circuits to be provided andthe distance over which they must give satisfactory trans mission. Thisdistance will be referred to as the range. The curve is prepared on thebasis of the product of the number of circuits and their length. Forexample. approximately of all the miles of toll circuits in Europeconsists of circuits each having a length less than 500 miles. Now thecost per mile of providing a circuit of a given range depends on thatrange; moreover, it is found as a result of practical experience thatthis cost is almost a function of the range alone in a well designedsystem, so that whatever range is required a circuit having a'cost permile approximately as indicated by a curve between cost per mile andrange canbe obtained, sometimes in several alternative ways. Such acurve drawn with range as abscissas and cost per mile of circuit asordinates is given in Fig. 2, in which A is the curve for side circuitsand B the curve for phantom circuits. It may be found that circuitsexceeding 3000 miles in length be best provided by means of somespecialized form of construction which would embody the use of screenedpairs (i. e. shielded pairs of conductors) with repeaters on a fairlylong spacing. According to the present invention the bulk of thecircuits in the system should be provided with a relatively fewdifferent types of circuits, say three. If the first type of circuit beused up to a range of X, the second for ranges between X and Y and thethird for ranges between Y and 3000 miles, and if as, y and 2 be thecost per mile of circuit of the three types of circuit, and if a, b andc be the percentages of the total miles of circuit provided by the threetypes of circuits, respectively, then as before mentioned the total costS=aa1+by+cz is a measure of the total cost of the system which should bea minimum.

The relative values of S for dilferent values of X, Y are plotted inFig. 3, in which the abscissas represent the range and the ordinates therelative cost for 100 circuits. Curve a represents relative cost for 100circuits when X=100 miles; curve 1) cost when X=150 miles; curve 0 costwhen X=200 miles; curve d when X=250 miles; curve a cost when X=300miles; and curve f cost when X=350 miles. These curves have beenprepared from actual statistics. It is seen from the curves that S is aminimum when X=250 miles (curve (I) and Y=550 miles (abscissa). Similarconstructive Figs. 4 and 5 represent similar curves plotted in terms ofannual charges, instead of first cost. In Fig. 4, curve A is the curvefor side circuits and B that for phantom circuits. It has been assumedso far for simplicity that the ranges of the phantom circuits are thesame as those of their respective side circuits although as a matter offact the range of the phantom circuits is somewhat greater for the firsttwo groups in most practical systems, and therefore much greater in thecase of the long distance group. If the range of the side circuits ofthe long distance group is 1250 miles, the number and range of thephantom circuits would be adequate to take care of a few circuits whichextend to 3000 miles. Due account of this has been taken in preparingthe curves. Consequently the ranges to be aimed at in the practicalsystem for the side circuits are 250, 550 and 1250 miles respectively.

Now, as more reliable data is now available regarding the constants tobe used in the formulae given in the above mentioned patent, it isdesirable to recapitulate these constants in order that the nature ofthe final solution may be appreciated. The following are the up to dateconstants The value of T is that for fourwire circuits assuming no echosuppressors. If echo suppressors are used,-higher values ,(say .060sec.) may be employed, but it is desirable that the above value benotgreatly exceeded in order that similar loading may be used for two wirecircuits (upon which it is difficult to use echo-suppressors) and inorder that a reasonable number of circuits may be able to operatewithout echo suppressors Using the above constants and the formulae inthe above mentioned patent, itis I found that the range of the loadingsystem using 177 n1.H coils on a spacing of 2000 yards, as at presentwidely adopted, is about 500 miles on a four wire basis. Thecorresponding range of 0.9 mm. two wire circuits with this loading isabout 300 miles. From this it would seem that 0.9 mm. two wire circuitsloaded with 177' m.H coils on a spacing of 2000 yards would be suitablefor the first type of circuit, while the same typeof circuit on a fourwire basis would be suitable for the second type.

Applyingthe formulae to the design of a 1250 mile side circuit thefollowing solution is obtained fc=2960 cycles per L =4.65 henriesspacing=224 miles second The phantom circuits having about the sameattenuation will have a range of about 3000 miles.

The solution thus approximates to the use of coils of half the usualinductance placed on a double spacing. The estimated range of the 1.3mm. two wire circuits using this type of loading is about 650 miles.There are thus two possible methods of providing circuits of the secondtype, with 0.9 mm. four wire circuits with 177 m.H coils, or 1.3 mm. twowire circuits with 83 IILH coils; the second method is more economicaland is preferred. It will be noted that the use of the double spacing isone feature of systems designed in the manner described.Existing'systems us-- ing 44 m.H coils on a 2000 yard spacing have arange of 5000 miles on a four wire basis with echo suppressors and 750miles on a two wire basis with correspondingly increased cost ascompared with the proposed system.

This is considered undesirable, particularly as it is not possible touse the full range of the four wire circuits owing to maintenancedifliculties. Other features are theabsolute values and the ratios ofthe ranges of the various types of circuits, which are considerablydifierent from systems so far designed.

The 83 m.Hcoils applied to the various long distance circuits wouldpreferably be staggered so that the number of 17 7 m.H

and 83 m.H coils contained in-each "loading coil-case wouldbe'approximately the same.

It istobe understood that slight changes.

ing coilswill he arrived at which will enable." the coils for each typeofcircuit to be located in the same pots. For instance, using T'forthe'timeof transmission per unit length and t for the building .uptimeper unit length, both'for the lowergrade or short distance circuits,some "value maybe adopted for the spacing on the high grade or longdistance circuits whichiwill giye the desired result. u

By giving the values to the time of transmission per unit lengthandbuilding up time per unit length respectively, of the higher gradecircuits, at being aconvenient whole number and ,beingequivalent to aused for the number of loading sections in theFormula (1) of the above-A mentioned. British patent, theloading coils willbe spaced apart oneachtype of; circuit in such a'mannerthat both types of coils will be inthe same pots. In choosing a value for n for the preferred embodiment ofthe invention, wherein double spacing is employed on the high gradecircuits, n will have a valueof2.. I i I ,-Whatis claimed is:- i

comprising two types of loaded toll cable circuits for transmission overrelatively long distances and relatively short distances, re-

spectively,the circuits intended for therelatively long distances.havingdouble the loading spacing ,of those for the relatively shortdistances, a

' 2. A-lo aded telephone transmission. sys:

tem comprising. lower. grade circuits having; a t me of transmissionperunit length of IT and buildingup time per unit length of t, andfcomprsing' h gher grade circuits whose lZlIl'lQ/ Of transmission per unitlength and bullding up time per unit length are respectively where n isa convenient number.

3. A loaded telephone transmission system A loaded telephonetransmission system comprising lower grade circuits having a time oftransmission per unit length 0 T and building up time per unit length oft, and comprising higher grade circuits whose time of transmission perunit length'and building up timeper unit length are respectively' andn nwhere n=2.

4. A loaded telephone transmission system comprising two types of loadedtoll cable circuits for transmission over relatively long distances andrelatively short distances, re-

spectively, the circuits intended for the relatively long distanceshaving double the load-.

ing spacing of those for the relatively short distances and the loadingcoils of the longer circuits being disposed at the alternate load ingpoints of the shorter circuits.

5. A loaded telephone transmission cable system comprising relativelyhigh grade toll circuits and relatively low, grade toll circuits inwhich the higher grade circuits are divided into two groups, the loadingpoints on one group occurring at even loading points on the lower gradecircuits and the loading points on the other group occurring at the oddloading points of said lower grade circuits.

6. A loaded telephone transmission system comprising two types of loadedtoll cable circuits for transmission over relatively long distances andrelatively short distances, respectively, the circuits intended for therelatively long distances having double the loading spacing of those forthe relatively short distances and the loading coils of the longercircuits having approximately half the inductance value of the coils inthe shorter circuits.

7. A loaded telephone transmission system comprising a plurality ofcircuits loaded with different loading spacings, the loading coilinductance varying as the reciprocal of the loading spacing.

8. A telephone transmission system comprising two types of phantomgroups of circuits for transmission over relatively long distances andrelatively short distances, respectively. and loading means for both theside circuits and the phantom circuits of said groups, the circuitsintended for the long distances having double the loading spacing ofthose for the short distances.

In witness whereof I hereunto subscribe my name this twelfth day ofFebruary, 1930.

KENNETH E. LATIMER.

